This invention relates to a magnetoresistive effect element, magnetic head and magnetic recording apparatus, and more particularly, to a magnetoresistive effect element structured to flow a sense current perpendicularly of the film surface of a magnetoresistive effect film, as well as a magnetic head and a magnetic reproducing apparatus using the magnetoresistive effect element.
Read-out of information recorded in a magnetic recording medium conventionally relied on a method of moving a reproducing magnetic head having a coil relative to the recording medium and detecting a current induced in the coil by electromagnetic induction then generated. Later, a magnetoresistive effect element was developed, and has been brought into practical use as a magnetic field sensor as well as a magnetic head (MR head) incorporated in a magnetic reproducing apparatus such as a hard disk drive.
For years, magnetic recording mediums have been progressively downsized and enhanced in capacity, and the relative speed between the reproducing magnetic head and the magnetic recording medium during information read-out operation has been decreased accordingly. Under the circumstances, there is the increasing expectation for MR heads capable of extracting large outputs even with small relative speeds.
As an answer to the expectation, it has been reported that multi-layered films, so called an “artificial lattice films”, which are made by alternately depositing ferromagnetic metal films and nonmagnetic metal films, such as the combination of Fe layers and Cr layers or the combination of Fe layers and Cu layers, under certain conditions, and bringing closely located ferromagnetic metal films into antiferromagnetic coupling, exhibit giant magnetoresistive effects (see Phys. Rev. Lett. 61 2474 (1988), Phys. Rev. Lett., vol. 64, p2304 (1990), for example). Artificial films, however, need a large magnetic field for magnetic saturation, and are not suitable as film materials for MR heads.
On the other hand, there are reports about realization of a large magnetoresistive effect by using a multi-layered film of the sandwich structure of a ferromagnetic layer on a nonmagnetic layer and a ferromagnetic layer even when the ferromagnetic layer is not under ferromagnetic coupling. According to this report, one of two layers sandwiching the nonmagnetic layer is fixed in magnetization beforehand by application of an exchanging bias magnetic field thereto, and the other ferromagnetic layer is magnetically reversed with an external magnetic field (signal magnetic field, for example). It results in changing the relative angle between the magnetization directions of these two ferromagnetic layers on opposite surfaces of the nonmagnetic layer, and exerting a large magnetoresistive effect. The multi-layered structure of this kind is often called “spin valve” (see Phys. Rev. B, vol. 45, p806 (1992), J. Appl. Phys., vol. 69, p 4774) (1981) and others).
Spin valves that can be magnetically saturated under a low magnetic field are suitable as MR heads and are already brought into practical use. However, their magnetoresistive variable rates are only 20% maximum. Therefore, to cope with area recording densities not lower than 100 Gbpsi (gigabit per square inch), there is the need of a magnetoresistive effect element having a higher magnetoresistance variable rate.
As its substitutional technique, a TMR (tunneling magnetoresistance) element has been proposed. The TMR element makes use of the phenomenon that spin-polarized electrons tunnel through an insulating barrier layer, and it exhibits an excellent magnetoresistance variable rate as high as 50% or more. However, to satisfy the magnetoresistance variable rate as high as 30%, for example, the area resistivity of the element becomes as high as 100 Ωμm2. Since reproducing heads for handling area recording densities not lower than 100 Gbpsi need downsizing the device area to a level not smaller than 0.1 μm2, resistance of the TMR element increases to 1 kΩ or higher, and results in decreasing S/N. To the contrary, if the resistance is lowered to about 10 Ωμm2, then the magnetoresistance variable rate also decreases to about 10%. Therefore, there is no clear prospect toward its practical use.
Structures of magnetoresistive effect elements are classified into CIP (current-in-plane) type structures permitting a sense current to flow in parallel to the film plane of the element and CPP (current-perpendicular-to-plane) type structures permitting a sense current to flow perpendicularly to the film plane of the element. Considering that CPP type magnetoresistive effect elements were reported to exhibit magnetoresistance variable rates as large as approximately ten times those of CIP type elements (J. Phys. Condens. Mater., vol. 11, p. 5717 (1999) and others), realization of the magnetoresistance variable rate of 100% is not impossible.
However, CPP type elements having been heretofore reported mainly use artificial lattices, and a large total thickness of films and a large number of boundary faces caused a large variation of resistance (output absolute value). To realize a satisfactory magnetic property required for a head, the use of a spin valve structure is desirable.
FIG. 16 is a cross-sectional view that schematically showing a CPP type magnetoresistive effect element having a spin valve structure. A magnetoresistive effect film M is interposed between an upper electrode 52 and a lower electrode 54, and a sense current flows perpendicularly to the film plane. The magnetoresistive effect film M shown here has the basic film structure sequentially made by depositing a base layer 12, antiferromagnetic layer 14, magnetization-fixed layer 16, nonmagnetic intermediate layer 18, magnetization free layer 20 and protective layer 22 on the lower electrode 54.
As these layers are made of metals. The magnetization-fixed layer (called pinned layer) is a magnetic layer in which magnetization is fixed substantially in one direction. The magnetization free layer 20 (called free layer) is a magnetic layer in which the direction of magnetization can freely change depending upon an external magnetic field.
This kind of spin valve structure, however, has a smaller total thickness and fewer boundary faces than those of artificial lattices. Therefore, if a current is supplied perpendicularly to the film plane, then the area resistivity AR becomes as small as the order of tens of mΩμm2. Of this resistance, the resistance of the active portion in charge of changes of the magnetoresistance is approximately 1 through 2 mΩμm2. As a result, even if the magnetoresistance variable rate is 50%, the area resistivity variable rate AΔR obtained is as small as 0.5 mΩμm2, approximately.